Filter tutorial?

[mlabbee]

I have found a number of articles describing basic filter design, but would like to get a bit more into the nuts and bolts of what needs to go into filter circuits - for instance, how to make variable filters, both manually variable ones (pots) and voltage controlled filters (i.e., LFO's, CV signals, etc.) and what needs to go around a filter to ensure the right impedances and keep the sound clean.

[mlabbee]

I should note that I have seen the TI tutorial, which has plenty on static active filters - I'm looking for info on variable filters.  I assume you have variable resistors in the appropriate locations, but how do you vary the value of a resistor without reaching down and twisting a pot?

[R.G.]

I assume you have variable resistors in the appropriate locations, but how do you vary the value of a resistor without reaching down and twisting a pot?

You've hit on one of the fundamental questions of effects - how do you change a resistance electronically?

There are several ways.

- LDR's (Light Dependent Resistors) which change resistance in response to light levels. Combine these with LEDs and bango! you have a variable resistor. Unfortunately, they're expensive, not matched, and LEDs eat a lot of current.
- JFET voltage variable resistance. JFETs act just like a voltage variable resistor in part of their characteristic. Unfortunately, they have a restricted current and voltage range, and add their own distortion to the signal as it passes through.
- Bipolar transistor resistance between collector and emitter; this varies, and can be used; however, it has a DC ofset voltage, is even more limited in voltage range than JFETs, and also adds distortion. Good for shunt-muting, though.

Many successful effects skip changing resistance and change something else.
- the Vox inductor wah changes gain  and thereby effective capacitance (see The Technology of Wah Pedals at GEO, http://www.geofex.com)
- some effects change the transconductance of an active circuit; see the articles on OTAs (Operational Transconductance Amplifiers) at GEO for some applications
- some odd wahs have changed capacitance (moving plates) or inductance (moving core in the inductor) but these have not been very successful